Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for separating up-going and down-going wave fields from seismic data recorded underwater or under the surface of the earth by a seismic receiver.
Discussion of the Background
Offshore and onshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys to more accurately decide where to drill in order to avoid a well with no or non-commercial quantities of hydrocarbons.
Marine and land seismic data acquisition and processing generate an image of the geophysical structure (subsurface). While this image/profile does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
For example, marine systems for the recording of seismic waves are based on towed streamers or on seafloor-deployed cables or nodes. An example of traditional marine system for recording seismic waves at the seafloor is illustrated in FIG. 1 and this system is described in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, plural seismic receivers 10 are removably attached to a pedestal 12 together with a memory device 14. Multiple such receivers are deployed on the bottom 16 of the ocean. A source vessel 18 tows a seismic source 20 that is configured to emit seismic waves 22 and 24. Seismic waves 22 propagate downward, toward the ocean bottom 16. After being reflected from a structure 26, the seismic wave (primary) is recorded (as a trace) by the seismic receiver 10, while the seismic waves 24 reflected at the water surface 28 are detected by the receivers 10 at a later time. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic or seismic waves), the reflected wave 24 travels back toward the receiver 10. This reflected wave is traditionally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the receivers 10, but with a different polarization and a time lag relative to the primary wave 22. As the primary wave 22 moves in an upward direction toward the receiver 10, this wave is sometimes called an up-going wave-field, and as the ghost 24 moves in a downward direction toward the receiver 10, this wave is sometimes called a down-going wave-field.
FIG. 1 also shows the receiver 10 being configured to detach from the pedestal 12 and to rise to the water surface 28 to be retrieved by a collecting boat 30. Based on the data collected by the receiver 10, an image of the subsurface is generated by further analyses.
As discussed above, every arrival of a marine seismic wave at receiver 10 is accompanied by a ghost reflection. The same applies for every arrival of a land seismic wave recorded by a buried receiver. In other words, ghost arrivals trail their primary arrival and are generated when an upward traveling wave is recorded a first time on submerged equipment before being reflected at the surface-air contact. Primary and ghost (receiver-side ghost and not the source-side ghost) signals are also commonly referred to as up-going and down-going wave-fields.
The time delay between an event and its ghost depends entirely upon the depth of the receiver 10 and the wave velocity in water (this can be measured and is considered to be approximately 1500 m/s). It can be only a few milliseconds for towed streamer data (depths of less than 15 meters) or up to hundreds of milliseconds for deep Ocean Bottom Cable (OBC) and Ocean Bottom Node (OBN) acquisitions. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches or gaps in the frequency content, and these notches cannot be removed without the combined use of advanced acquisition and processing techniques.
Such advanced processing techniques include wave-field separation or wave-field decomposition or deghosting. These techniques require advanced data acquisition, i.e., multi-component marine acquisition. Multi-component marine acquisition uses receivers that are capable of measuring at least two different parameters, for example, water pressure (recorded with a hydrophone) and water particle acceleration or velocity (recorded with a geophone or accelerometer). Thus, multi-component marine acquisitions deliver, besides a pressure recording P, at least a vertical particle velocity (or acceleration) component Z.
A sensitive data-processing step for marine multi-component recordings is pre-stack wave-field separation. Wave-field separation allows the separation of the recorded wave-field into its individual parts: up-going and down-going waves. Various techniques are known in the field for wave-field separation, e.g., Amundsen, 1993, Wavenumber-based filtering of marine point source data, Geophysics; or Ball and Corrigan, 1996, Dual-sensor summation of noisy ocean-bottom data, SEG Ann. Mtg.; or Schalkwijk et al., 2003, Adaptive decomposition of multi-component ocean-bottom seismic data into downgoing and upgoing P and S waves, Geophysics, the entire contents of which are incorporated herein by reference.
Regardless of the type of separation and of the details of the algorithm used, current separation algorithms assume that the recording surface is a planar surface. However, the ocean bottom is a non-planar acquisition surface. Alternatively, the towed-streamer depth may vary along its length, or buried receivers may be deployed at variable depth. Thus, for these situations, the planar surface assumption fails, and the collected data may generate spurious effects in the final image unless it is corrected.
Accordingly, it would be desirable to provide systems and methods that avoid the aforedescribed problems and drawbacks, e.g., take into account the non-flat acquisition surface.